The present invention relates to molding apparatus for use in a process for preparing thermoplastic resin molded articles, especially thermoplastic expansion-molded articles, or molded articles of thermoplastic resin having a high melt viscosity and difficult to mold in a molten state.
Expansion-molded articles of thermoplastic resin can be prepared by a process wherein a chemical blowing agent is used (for chemical expansion) or by a process wherein a gas, such as a chlorofluorocarbon, butane, pentane, carbon dioxide or nitrogen, is directly supplied to and dissolved in the resin for use as a blowing agent (for physical expansion). In recent years, there is a growing demand for producing expansion-molded articles of thermoplastic resin with use of carbon dioxide, nitrogen or like inert gas for physical expansion in consideration of sanitation and environment.
In view of the above situation, many studies have been made on processes for preparing expansion-molded articles of thermoplastic resin with use of inert gases (see, for example, JP-A No. 10-230528/1998).
However, since many of the conventional processes intend to solve the problem of how to produce expansion-molded articles of fine cells, they require very complex equipment comprising a plurality of devices. To realize physical expansion, there is a need to use a cylinder, screw and controller which are designed specially, in the gas impregnation step of melting a thermoplastic resin, supplying an inert gas to the molten resin and impregnating the resin with the gas by mixing. The equipment then requires devices which are made anew or substantial modification of existing devices to result in the problem of an increased cost.
In view of the foregoing problem of the prior art, an object of the present invention is to provide a molding apparatus which is adapted to realize a gas impregnation step by continuously supplying carbon dioxide, nitrogen or like inert gas to a thermoplastic resin at a relatively low pressure with good stability and using a screw having such an overall length as to permit the use of, an existing molding machine cylinder as it is.
We have conducted research not from the viewpoint of how to produce expansion-molded articles of very fine cells but from the viewpoint of how to realize physical expansion with use of an inert gas, such as carbon dioxide or nitrogen, with the greatest possible ease and at as low a cost as possible. Consequently, we have realized physical expansion easily at a low cost by causing a screw to perform all the functions of melting a thermoplastic resin, supplying an inert gas to the molten resin and mixing the gas with the molten resin for impregnation, utilizing existing injection molding cylinder, controller, etc. as they are.
The molding apparatus of the invention for preparing molded articles of thermoplastic resin is an apparatus for use in practicing a molding process for producing molded articles of thermoplastic resin which comprises the gas impregnation step of supplying an inert gas from a gas supply opening in a screw to a thermoplastic resin melted by the rotation of the screw within a molding machine cylinder to impregnate the molten resin with the inert gas, and the molding step of obtaining an expansion-molded article from the gas-impregnated molten resin, the apparatus being characterized in that the screw for use in the gas impregnation step comprises a resin melting portion positioned in an upstream region of the cylinder for rendering the resin molten, a molten resin nonfilled-up portion integral with a downstream side of the resin melting portion for making the resin lower than the inert gas at the gas supply opening in pressure, and a gas impregnation portion integral with a downstream side of the molten resin nonfilled-up portion for supplying the inert gas and impregnating the molten resin with the inert gas.
The molten resin nonfilled-up portion of the screw is a portion formed by reducing the screw shaft diameter or increasing the flight pitch thereof. The provision of the molten resin nonfilled-up portion produces an enlarged space defined by the screw fight, the cylinder and the screw shaft, whereby the resin within this space can be made lower than the gas at the gas supply opening in pressure. The molten resin nonfilled-up portion extends from the downstream end of the resin melting portion to a position downstream therefrom where the space defined by the screw fight, the cylinder and the screw shaft is greatest. The space defined by the screw fight, the cylinder and the screw shaft is smaller in the gas impregnation portion extending from the downstream end of the nonfilled-up portion than in the molten resin nonfilled-up portion, so that the impregnation portion becomes gradually filled up with the molten resin which used to be in the nonfilled-up state. Thus, the gas impregnation portion has two states as shown in FIG. 4, i.e., a molten resin nonfilled-up state in the upstream region and a molten resin filled-up state in the downstream region. Since the screw has the gas supply opening in the region of such a molten resin nonfilled state, a required amount of the inert gas is supplied to the molten resin with good stability.
If an attempt is made to additionally provide the gas impregnation portion at the forward end of the screw of the conventional apparatus, the screw becomes greater in overall length by an amount corresponding to the added portion, and the cylinder of an existing molding machine can not be utilized, whereas the construction of the invention described above makes it possible to provide the gas impregnation portion while enabling the screw to retain such an overall length as to permit the use of the cylinder of an existing molding machine.
In the molding apparatus described, the screw preferably has a gas inlet channel formed in the upstream end thereof, and a gas supply channel communicating with the gas inlet channel and extending through the screw longitudinally thereof, the gas supply opening being formed in the gas impregnation portion and communicating with the gas inlet channel via the gas supply channel. It is then possible to use an existing cylinder without making almost any modification therein, that is, without modifying the gas inlet, gas supply channel and gas supply opening thereof.
The resin melting portion of the screw comprises a solid (hereinafter xe2x80x9csolidxe2x80x9d means powder, pellets or the like) transport part disposed at an upstream position and having a small screw shaft diameter, a molten resin transport part disposed a downstream position and having a large screw shaft diameter, and a compression melting part positioned between the two parts and having a screw shaft diameter gradually increasing downstream, the lengths of the screw parts and portions preferably having the following relationships with the cylinder diameter D:
length L1 of the solid transport part=5D-10D,
length L2 of the compression melting part=3D-6D,
length L3 of the molten resin transport part=1D-4D,
length L4 of the molten resin nonfilled-up portion=0.1D-2D, and
length L5 of the gas impregnation portion=4D-10D. The resin can then be plasticized with heat and also with the inert gas which affords an additional plasticizing effect, with the result that the molten resin is impregnated with the inert gas positively and plasticized more effectively with the inert gas. The plasticizing effect produced by the inert gas refers to the phenomenon shown in FIG. 3 that molecules of the inert gas (carbon dioxide in the illustration) dissolving in the resin between molecular chains thereof expand the spaces between the molecular chains, consequently increasing the free volume of the molecular chains to plasticized the resin substantially in the same manner as by the plasticization with heat.
The solid transport part is given a length L1 of 5D to 10D (optimally about 8D) because a diminution in the solid transport part due to the metering stroke is considered in designing the resin melting portion. If L1 is smaller than 5D, it becomes impossible to transport the resin in the form of unmelted pellets or powder from the hopper with good stability, whereas if L1 is greater than 10D, the screw has an increased overall length, presenting difficulty in utilizing the existing cylinder.
The compression melting part is given a length L2 of 3D to 6D (optimally about 4D) because if L2 is smaller than 3D, a satisfactory molten state is not available, and further because if L2 is greater than 6D, the screw has an increased overall length.
The molten resin transport part is given a length L3 of 1D to 4D (optimally about 2D) because if L3 is smaller than 1D, it is impossible to prevent the inert gas from leaking into the resin supply hopper, and further because if L3 is greater than 4D, the screw has an increased overall length. To suppress variations in the resin pressure at the downstream end of the molten resin transport part and to ensure promoted melting, it is generally desirable that L3 be greater, whereas it is only required according to the present invention to prevent the inert gas at the downstream end of the molten resin transport part from leaking into the resin supply hopper. The desired performance is fully available if L3 is up to 4D.
The molten resin nonfilled-up portion is intended to provide a molten resin nonfilled state to ensure stabilized supply of the inert gas. Satisfactory performance can be achieved if the length L4 thereof is up to 2D (preferably up to about 1D). If L4 is greater than 2D, the screw has an increased overall length.
The gas impregnation portion comprises a tapered part gradually increasing in screw shaft diameter from the downstream end of the molten resin nonfilled-up portion, and a solid cylinder part downstream from this part and having a constant screw shaft diameter. Preferably, the length L6 of the tapered part has the relationship of L6=0.5D-3D with the cylinder diameter D. The molten resin sent from the resin melting portion (where the space defined by the screw flight, the cylinder and the screw shaft is smallest) to the molten resin nonfilled-up portion (where the space is greatest) is delivered to the tapered part where the space defined by the screw flight, the cylinder and the screw shaft is smaller than in the nonfilled-up portion and then to the solid cylinder portion where the space is greater than in the resin melting portion. Consequently, the molten resin in the gas impregnation portion has two states as shown in FIG. 4, i.e., a nonfilled-up state in the tapered part and the upstream region of the solid cylinder part and a filled-up state in the downstream region of the solid cylinder part. The molten resin is made to have two states in order to realize stabilized supply of the gas in the molten resin nonfilled-up state, and prevent a discharge of the gas due to a leak to the injection nozzle in the molten resin filled-up state.
Preferably the length L5 of the gas impregnation portion is 4D to 10D (optimally about 7D). If L5 is smaller than 4D, it is impossible to prevent the gas from being forced out due to the leak of the gas to the injection nozzle, whereas if L5 is greater than 10D, the screw will have an increased overall length. It is desired that the length L6 of the tapered part be at least 0.5 to up to 3D to stabilize the molten resin nonfilled-up state and the molten resin filled-up state.
By giving the above construction to the screw, the screw can be caused to perform all the functions of melting a thermoplastic resin, supplying an inert gas to the molten resin and impregnating the molten resin with the gas by mixing. Moreover, since the screw can be designed with a short overall length, existing injection molding cylinder and controller can be utilized. Thus, physical expansion can be realized easily at a low cost by using the screw having the above functions.
The molding machine of the present invention is not limited to use for injection molding but is applicable also to extrusion molding, blow molding, injection blow molding, film forming, etc.
The thermoplastic resin for use in the present invention is not limited particularly. Examples-of such resins are resins having a high melt viscosity and therefore difficult to mold in a molten state, resins easily decomposable thermally, resins containing a low-boiling-point additive or an additive easily decomposable thermally and difficult to mold, etc.
Examples of resins having a high melt viscosity and therefore difficult to mold in a molten state are ultra-high-molecular-weight polyethylene, polyvinyl chloride having an ultrahigh degree of polymerization, polytetrafluoroethylene, polyimide and like resins for use as engineering plastics.
Examples of resins easily decomposable thermally are polylactic acid, polyhydroxybutyrate and like biodegradable resins, polyvinyl chloride having a high degree of chlorination, polyacrylonitrile, etc.
The inert gas for use in the present invention is not limited specifically insofar as the gas is nonreactive with the resin and does not degrade the resin or produces no adverse effect thereon. Examples of such gases are carbon dioxide, nitrogen, argon, neon, helium, oxygen and like inorganic gases, chlorofluorocarbons, low-molecular-weight hydrocarbons and like organic gases.
Preferable among these gases are inorganic gases because they are less likely to exert an adverse effect on the environment and need not be collected after use. Carbon dioxide is more preferable from the viewpoint that the gas is highly soluble in resins which are difficult to mold, highly effective for melting the resin and releasable directly into the atmosphere almost without causing any harm. Inert gases may be used singly, or at least two kinds of gases are usable in combination.